Currently the top holder is EVE online for it's massive exploration in multiple systems and community social orders. You can go back to Wing Commander for being the top break out space sim for its day. It would be eclipsed by Xwing and then Tiefighter. Microsoft had a bevy of space sims next to Jane's military titles. Right now I'd check out Space Engineers. It has the potential for being a run away hit due to its Minecraft roots. You've seen the promo for No Man's Sky and that has the strongest chance of meeting both the Space Fighter and Exploration component that many space sim players are looking for. Star Citizen as it progresses along with crowd funding also has the potential of being one of the top Space sims since Freelancer. Even Spore when you get past the cute alien horrors that were made by its community is actually on paper a good space exploration and empire builder, while affording the player a certain window into their own generated content. There's plenty of other examples. Personally I believe if the Developer can give the players the tools to create their own ships and adventures in procedural generated universes they will see a stronger interest in space sims. You might also want to check out some of the earlier builds of Infinity quest for earth. They had some nice footage on entering and exiting atmosphere into space and gives a window into the possibilities of exploration.